Up to now, a twin-clutch type transmission (1) has been provided, as shown in FIG. 3, having a first input shaft X1 connecting to a first clutch D1, a second input shaft X2, inserted in said first input shaft X1, connecting to a second clutch D2, an output shaft Xo, a counter shaft Xs, and an idle shaft XR, wherein 4th and 6th speed drive gears G18, G19 are rotatably carried on said first input shaft X1, the 2nd drive gear G20 is fixedly carried on said first input shaft X1, and a coupling C5 to fix selectively said rotatable gears G18, G19 to said first input shaft X1 is attached between said two gears G18, G19, the 1st, 3rd, and 5th drive gears G21, G22, G23 are fixedly carried on said second input shaft X2, the 4th and 6th speed driven gears G24, G25 engaging respectively with said rotatable two gears G18, G19 of said first input shaft X1 are fixedly carried on said output shaft Xo, the 1st and 5th speed driven gears G26, G27 engaging respectively with said two fixed gears G21, G23 of said second input shaft X2 are rotatably carried on said output shaft Xo, an output gear G12 connecting to a differential gear(4) is fixedly carried on said output shaft Xo on the clutch-most side, and a coupling C6 to fix selectively two rotatable gears G26, G27 to said output shaft Xo is attached between said two gears, the 3rd speed driven gear G28 engaging with said gear G22 of said second input shaft X2 and a gear G30 connecting to said gear G21 of said second input shaft X2 through a gear G29 carried fixedly on said idle shaft XR, are respectively rotatably carried on said counter shaft Xs, and a gear G31 engaging with said gear G18 of said first input shaft is fixedly carried on said counter shaft Xs, and the 2nd speed driven gear G32 engaging with said gear G20 of said first input shaft X1 is rotatably carried on said counter shaft Xo, furthermore, couplings C7, C8 to fix selectively three rotatable gears G28, G30, G32 to said counter shaft Xs are respectively attached between said gears G28 and G30, and to said gear G32.
In above described transmission (1), said gear G32 and said coupling C8 attached to said gear G32 are arranged in a side space SP of said output gear G12 of said output shaft Xo.
In said conventional transmission (1), to attain 2nd speed or 3rd speed or reverse, the 4th drive gear G18 of said first input shaft X1 is used as an idle gear, so that excessive load effects on said gear G18, and the mechanical strength of the bearing part of said gear G18 is insufficient, resulting in the degradation of the durability of said transmission (1).
Accordingly, to ensure the mechanical strength of the bearing part of said gear G18, increasing the diameter of the bearing part of said gear G18 to increase volume of the bearing part of said gear G18, has been considered, however, since the gear diameter of said gear G18 is settled by the gear ratio of the 4th drive gear G18 to the driven gear G24, in a case where the gear diameter of said gear G18 is settled to be large, the distance between said first input shaft X1 and said output Xo also increases, resulting in an oversized transmission, which is troublesome to install in an automobile. Furthermore, in a case where the shaft length of the bearing part of said gear G18 is enlarged to increase the volume of the bearing part of said gear G18, the shaft direction size of said transmission (1) is also enlarged, according to any enlargements of shaft length, thus causing trouble. In the case of twin clutch type transmission (1), particularly in the case of a front wheel drive system where the engine is installed sideways, narrowing the transmission installation space, the shaft length is difficult to accommodate in the space available.
Further, since the 2nd speed gear G20 of said first input shaft X1 is for a lower speed gear, the diameter of said gear is settled to be small, and the diameter of said first input shaft X1 is settled to be small, according to the diameter of said gear G20. Accordingly, the diameter of said second input shaft X2, inserted in said hollow, tube-like first input shaft X1, is necessary to be settled to be smaller, resulting in its mechanical strength being insufficient to degrade the durability of said transmission (1).
In this case, the diameter of said gear G20 is settled by the gear ratio of 2nd drive gear G20 to said driven gear G32, and in the case where the diameter of said gear G20 is enlarged, increasing the distance between said first input shaft X and said counter shaft Xs, the result is an enlarged transmission size, which is troublesome to install in an automobile, given the space provided therefor.